Carbazole/iridium dendrimer side-chain phosphorescent copolymers for efficient light emitting devices

Dendrimers have proved to be successful materials for solution-processable phosphorescent organic light emitting diodes (OLEDs). Recently we have been investigating poly(dendrimer)s with improved viscosity for ink-jet printing applications. In this work we present three phosphorescent side-chain copolymers comprised of a poly(styrene) backbone, carbazole charge transporting units, and iridium(III) complexes containing no dendrons (simple complex), a single dendron attached to two of the ligands, or doubly dendronised complexes with two dendrons added to the ligands. In addition to their charge transporting capability the copolymer structure incorporating carbazole units can also prevent aggregation of the chromophores, as evidenced by there being only one emissive environment in time-resolved luminescence. We report high external quantum efficiencies of 11.0% (37.3 cd A(-1)) at 100 cd m(-2) and 8.3 V for devices made with the neat copolymers. The best performance is achieved by blending the doubly dendronised copolymer with 50 wt% of 4,4'-bis(N-dicarbazolyl)biphenyl (CBP) which results in 14.7% EQE (48.3 cd A(-1)) at 100 cd m(-2) and 9.3V.